Problem 72 - Chemistry Matter and Change

Question

Soap Draw the structure of the soap sodium palmitate (palmitate is the conjugate base of the 16 carbon saturated fatty acid, palmitic acid). Label its polar and non polar ends.

Step-by-Step Solution

Answer

The structure of sodium palmitate is CH3-(CH2)14-COO- Na+, where the polar end consists of the carboxylate group (COO- Na+), and the nonpolar end consists of the hydrocarbon chain (CH3-(CH2)14). The polar head is hydrophilic and can form ionic and hydrogen bonds with water, while the nonpolar tail is hydrophobic and does not interact with water molecules.

Step 1: Draw the structure of palmitic acid

Palmitic acid (hexadecanoic acid) is a saturated fatty acid with a 16-carbon chain and a carboxylic acid group at one end. Its molecular formula is C16H32O2. The structure of palmitic acid is: CH3-(CH2)14-COOH

Step 2: Identify the conjugate base of palmitic acid (sodium palmitate)

In a basic solution or when reacting with a base, palmitic acid will lose a proton (H+) from the carboxylic acid group (COOH) to form its conjugate base, palmitate. When it reacts with sodium hydroxide (NaOH), sodium palmitate is formed. The reaction is: Palmitic Acid + NaOH → Sodium Palmitate + H2O CH3-(CH2)14-COOH + NaOH → CH3-(CH2)14-COO- Na+ + H2O

Step 3: Draw the structure of sodium palmitate

Now that we have the chemical formula of sodium palmitate, we can draw its structure: CH3-(CH2)14-COO- Na+ Here, the COO- group is attached to a sodium ion (Na+).

Step 4: Label the polar and nonpolar ends

In the sodium palmitate structure, the polar end is the carboxylate group (COO- Na+) which is capable of forming ionic and hydrogen bonds with water. The nonpolar end consists of the hydrocarbon chain (CH3-(CH2)14) which is hydrophobic and does not interact with water molecules. In summary, sodium palmitate's structure consists of a polar head (carboxylate group) which is hydrophilic, and a nonpolar tail (hydrocarbon chain) which is hydrophobic.

Key Concepts

Conjugate Base

When dealing with acids and bases, the term "conjugate base" is essential to understand. A conjugate base is what you get when an acid donates a proton (\( H^+ \)) during a reaction. In this case, when palmitic acid (a fatty acid) loses its proton, it turns into palmitate, which is the conjugate base.
The reaction can be represented as: \[\text{Palmitic Acid} + \text{Base} \rightarrow \text{Palmitate (Conjugate Base)} + \text{Water}\]Once the acid has given away its proton, the remaining molecule can now accept a proton from another substance if needed, exhibiting slightly basic traits.

Fatty Acid Structure

Fatty acids are fascinating compounds, composed of long hydrocarbon chains. They generally have two distinct parts: a hydrocarbon tail and a carboxylic acid group. The structure of palmitic acid, a saturated fatty acid, displays this perfectly.

• Hydrocarbon Chain: Consisting of 16 carbon atoms, arranged in a linear fashion, each carbon is saturated, meaning it's fully bonded with hydrogen atoms.
• Carboxylic Acid Group: Found at one end of the chain, this group is where the real action happens in biochemical reactions.

This dual characteristic explains why fatty acids like palmitic acid play a significant role in forming soaps such as sodium palmitate.

Polar and Nonpolar Ends

In chemistry, especially in organic compounds, understanding polar and nonpolar regions is crucial.
The concept mainly involves molecular structure and how these regions interact with water:

• Polar End: This is the section that can dissolve in water due to its charge. In sodium palmitate, the polar end is the carboxylate group \( ext{COO}^- ext{Na}^+ \), which can form bonds with water molecules.
• Nonpolar End: This part does not interact well with water, remaining separate. In sodium palmitate's case, the nonpolar region is made up of the lengthy hydrocarbon chain \( ext{CH}_3( ext{CH}_2)_{14} \).

These characteristics are why soaps can clean, with the polar end attracting water and dirt, while the nonpolar end captures oils and grime.

Carboxylate Group

The carboxylate group, \( - ext{COO}^- \), plays a pivotal role in chemistry because it introduces polarity into a molecule. It results from the deprotonation of a carboxylic acid group \( - ext{COOH} \).
In sodium palmitate, once the carboxylic acid loses its hydrogen, it forms the negatively charged carboxylate ion that readily pairs with sodium ions \( ext{Na}^+ \).
This ionic interaction is also what makes the carboxylate group hydrophilic or water-loving, allowing it to dissolve in water and interact with other ionic materials or polar compounds, which is essential in soap applications.

Hydrophilic and Hydrophobic Properties

One intriguing property of many molecules is their interaction with water, categorized into hydrophilic (water-loving) and hydrophobic (water-repelling) components.
In sodium palmitate:

• Hydrophilic Head: The negatively charged carboxylate group \( ext{COO}^- ext{Na}^+ \) is the hydrophilic part, easily bonding with water, allowing the soap to disperse in it.
• Hydrophobic Tail: This is the extensive hydrocarbon tail \( ext{CH}_3( ext{CH}_2)_{14} \), which avoids water and instead interacts with oils and grease.

These properties are key in soap's ability to clean, where the hydrophilic heads latch onto water while the hydrophobic tails capture oily residues, effectively removing dirt from surfaces.